Autoimmune (idiopathic) thrombocytopenic purpura (ITP), a disease caused by auto-antibodies directed against platelets, has been difficult to demonstrate for a variety of reasons. In general, when the antibodies can be demonstrated, there is an inverse correlation with the platelet count. We are setting up two assays, one screening assay and one more specific assay, to aid in the diagnosis and progress of treatment for patients with ITP. We will particularly utilize the tests for following patients pre- and post-treatment in a pilot study with NHLBI in the treatment setting of T-cell-depleted auto-stem cell transplantation in patients with severe ITP.